The present invention relates to a windowpane antenna apparatus for vehicles which is mounted on a windowpane of a vehicle such as an automobile.
There is a windowpane antenna apparatus for automobiles as the most typical one of conventional windowpane antenna apparatuses for vehicles. The typical antenna apparatus includes a thin, narrow, strip conductor provided on a window (usually a rear window) of an automobile, and the strip conductor is employed as an antenna.
In recent automobiles, a defogger is provided almost all over the rear window to serve as a heater for defogging the window. The antenna therefore has to be mounted in a limited space between the defogger and the window frame.
FIG. 8 shows an example of a prior art automobile windowpane antenna apparatus. As shown, a defogger 110 is mounted on a rear window 100, and a loop-shaped antenna 120 constituted of a strip conductor is formed in a region above the defogger 110.
A DC power supply voltage is applied to the defogger 110 from a car-mounted battery 111 through a noise filter 112 (which is constituted of, e.g., a choke coil and a capacitor) for eliminating high-frequency noise (in the AM band) and a power supply voltage application line 113.
A reception signal of the antenna 120 is transmitted to a receiver set such as a radio from a feeding point 121 through a feeding cable (not shown).
FIGS. 9 to 11 are illustrations for explaining the performance of the prior art automobile windowpane antenna apparatus described above. These illustrations are used to describe an automobile windowpane antenna apparatus according to an embodiment of the present invention in comparison with the prior art apparatus.
The prior art antenna apparatus has the problem that its reception sensitivity in the AM and FM bands is not obtained sufficiently since a space for mounting the antenna 120 is limited. The apparatus also has the problem that since frequency characteristics are not flattened within a receiving band, tuning for optimizing the reception performance is difficult and a long period of time is required for performing the tuning operation.